The present invention generally relates to apparatus and methods for end fittings for pressure vessels.
A cylindrical pressure vessel has a hoop stress that is twice the longitudinal stress. Therefore a number of technologies exist for reinforcing pressure vessels by strengthening them in the hoop direction by the application of wires wound helically onto the cylindrical pressure vessel or by applying plastic reinforced by fibers in the hoop direction. By these technologies a pressure vessel is obtained that has a lower weight.
End fittings are usually comprised of hemispherical caps or ellipsoidal caps. The end fittings may also be reinforced by the material reinforcing the cylindrical pressure vessel in the hoop direction. The application of the reinforcing to the end fitting presents special problems in terms of applying the reinforcing in the proper geometry and quality. The application of the reinforcing to the end fitting is a difficult and expensive process.
Many reinforced cylindrical pressure vessels are therefore fitted with unreinforced end fittings. When the pressures are high and the diameter of the cylindrical pressure vessel is large the material thickness of the hemispherical caps or ellipsoidal caps becomes large and the cap is difficult and expensive to manufacture.
An example of such a pressure vessel is the tank marketed by the Floating Pipeline Company of Newfoundland Canada. This tank is designed to hold compressed natural gas at a pressure of approximately 23 MPa. It is 915 mm in diameter, made from steel and reinforced by glass reinforced plastic in the hoop direction. The steel wall thickness is approximately 35 mm permitting a hemispherical end fitting made from the same steel with the same wall thickness to safely contain the pressure without being reinforced.
FIG. 1 shows a sectional view, in a plane containing the rotational axis of symmetry, of a prior art cylindrical pressure vessel 6 comprised of a metal cylinder 1 reinforced on the outside by a layer of reinforcement 2 which may be glass-reinforced plastic. The pressure vessel is fitted with a hemispherical end fitting 4 made from the same material as used for the cylinder 1. The end fitting 4 also has the same wall thickness as the cylinder 1. The reinforcement layer 2 reinforces the pressure vessel 6 in the hoop direction only. If the reinforcement layer 2 resists 50% of the internal pressure in pressure vessel 6 then the metal cylinder 1 will have equal hoop and longitudinal stresses, which in turn will be equal to the tensile stresses in the hemispherical end cap 4. The reinforcement layer 2 may be wound onto a cylindrical extension 3 to the pressure vessel 6 in order to ensure that a weak point does not develop at the weld seam 5 between end cap 4 and cylinder 1.
Current rules governing the storage of natural gas in pressure vessels require a shut-off valve and a safety device for each individual tank or pressure vessel. If a 36 inch or 40 inch diameter pressure vessel is used, only one shut-off valve and one safety device may be needed. If, for example, one wanted to store nine times as much natural gas as that stored in a pressure vessel with a 36 inch diameter pressure vessel, one would need nine tanks, each with a safety device and a shut-off valve. A single pressure vessel having a diameter of 108 inches would be able to store nine times the natural gas stored in a single 36 inch diameter and such a tank would only require a single shut-off valve and a single safety device under current rules. Although it would be advantageous to use the much larger diameters in tanks that store natural gas under high pressure in order to reduce the number of tank connections with associated safety devices and valves, practical and cost limitations prevent this. For example, it is exceedingly expensive to make an end cap that is 108 inches in diameter. In fact, not that many places in the world can even manufacture such an end cap since it requires specialized heavy equipment and special handling.
The difficulty of using thicker material for the end cap of a pressure vessel is further illustrated by reference to another class of cylindrical pressure vessels, which are reinforced on the exterior by high strength steel wires. These wires may be laid with a lay angle such that they also reinforce the tank in the longitudinal direction. Such tanks may have a particularly thin steel wall. This in turn creates a problem of stress continuity at a conventional end fitting having a much thicker wall.
As can be seen, there is a need for a cost effective and practical apparatus and method for a large diameter high pressure vessel, particularly one that is generally cylindrical.